1. Field of the Invention
The present invention relates to a multi-layer printed board which can reduce electromagnetic inductive interference due to power source current, when a multiplicity of high-speed and high-frequency circuit elements are mounted thereon.
2. Description of the Related Art
It is well known that with multi-layer printed boards on which are mounted high-speed and high-frequency circuit elements such as ICs (Integrated Circuits), LSIs (Large Scale Integrated circuits), that there is a problem in that since electromagnetic noise occurs, electronic equipment mounted on the printed board or other electronic equipment is subjected to EMI (Electro Magnetic Interference), thereby causing malfunction.
Of this EMI, one of particularly great significance is electromagnetic noise resulting from an RF source, caused by a large ground or a ground face used as a reference potential, also referred to as xe2x80x9ccommon mode noisexe2x80x9d. However, the presumed source of the common mode noise is diversified, and respective source mechanisms are complicated. Hence there has heretofore been no effective method for remedy in the vicinity of the source of release. Therefore, only measures for preventing leakage or radiation of the common mode noise to a cable serving as a main propagation path or as a radiation antenna, have heretofore been taken.
On the other hand, recent research has revealed the fact that one of the largest causes of the common mode noise in high-speed digital circuits is power source current to high-speed and high-frequency circuit elements mounted on a printed board. As an invention developed based on this fact, there is for example, the technique registered by Japanese Patent (Granted) Publication No. 273447 and the technique applied for in Japanese Patent Application, First Publication No. Hei 9-253519.
These technique are for supplying a DC power source with respect to high-speed and high-frequency circuit elements mounted on a printed board by using power source wiring in which an inductance element exhibiting a high impedance at the time of high frequencies, is inserted in the middle of a line. Alternatively this is done by using a power supply line in which characteristic impedance is made high by surrounding the periphery of the line with a magnetic body, as well as connecting a capacitor between the power source of circuit elements and the ground. As a result, while enabling smooth high speed and high frequency operation of circuit elements mounted on the printed board, high frequency power source current generated by the operation is prevented from diffusing over the printed board.
Studies confirming that the electromagnetic radiation level is largely suppressed by applying these techniques to high-performance computers, and that resistant strength (immunity) against electric or electromagnetic disturbance from the outside is improved, are typically published in, for example, xe2x80x9cMagnetic body built-in decoupling reinforced multi-layer printed boardxe2x80x9d (Institute of Electrical Engineers, Magnetics Research Society; 1997-12), xe2x80x9cNovel decoupling circuit enabling notable electromagnetic noise suppression and high-density packing in a digital printed circuit boardxe2x80x9d (IEEE International Symposium on Electromagnetic Compatibility; 1998-8, Denver).
In the above described background art, a line structure wherein an impedance of a DC power supply line of the printed board is made high in a high frequency domain (hereinafter referred to as xe2x80x9cdecoupling inductorxe2x80x9d) is adopted, and a capacitor (hereinafter referred to as xe2x80x9cbypass capacitorxe2x80x9d) is used for efficiently splitting the high frequency power source current generated with the high speed and high frequency operation of circuit elements.
Below is a description of an example from Japanese Patent No. 273447, as background art known to have a noticeable EMI suppression effect.
FIG. 5 is a sectional view of a printed board in the prior art, FIG. 6 is a plan view showing a power source layer in the printed board in the prior art, FIG. 7 is a diagram showing an equivalent circuit (decoupling circuit) of a power supply circuit to which the prior art is applied, and FIG. 8 is a diagram for explaining a diffusion suppression effect of a high frequency power source current in a printed board to which the prior art is applied.
The printed board in the prior art comprises, as shown in the sectional view in FIG. 5, a power source layer 101, ground layers 102, signal layers 103, magnetic insulation layers 104 and dielectric insulation layers 105, and in order of from the top to the bottom, there are formed the signal layer 103, the dielectric insulation layer 105, the ground layer 102, the magnetic insulation layer resistant strength, the power source layer 101, the magnetic insulation layer 104, the ground layer 102, the dielectric insulation layer 105, and the signal layer 103.
Here, the magnetic insulation layers 104 comprise an insulating material in which a magnetic body is incorporated, and the dielectric insulation layers 105 comprise an insulating material having only a dielectric property.
Moreover, in the power source layer 101 in a printed board in the prior art, as shown in a plan view of FIG. 6, there are arranged a main wiring 106 and a branch wiring 107 branched from the main wiring 106, and IC/LSIs 108 fitted to a parts face (for example, the surface of the signal layer 103) of the printed board are connected to the point of the branch wiring 107 through a wire hole (not shown), and a decoupling capacitor 109 fitted to a parts face (for example, the surface of the signal layer 103) of the printed board is connected to a connection portion between the branch wiring 107 and the IC/LSIs 108, respectively.
In the printed board of the prior art, as shown in FIG. 7, an equivalent circuit of the power supply circuit for each IC/LSI connects a power source III to an IC/LSI 110 via a power source wiring 112, and connects the IC/LSI 110 and a return circuit of the power source III to a ground layer 113.
At this time, since the magnetic insulation layers 104 are disposed above and below the power source layer, impedance of the power source wiring formed in the power source layer becomes high, becoming equivalent to the case where the decoupling inductor (L) 114 is inserted, as shown in FIG. 7. The inductance 114 and the capacitance (C) 115 of the decoupling capacitor form a low pass filter. Hence, with operation of the IC/LSI 110, the high frequency power source current flowing in the power supply line is suppressed. Moreover, the construction may be such that the decoupling conductor is increased by using an impedance added circuit comprising a meandering portion or the like, for the power source wiring 112.
In this prior art, as seen from the respective figures, with operation of the IC or LSI, the high frequency power source current flowing into the power source layer is blocked by the inductor inserted in the wiring structure, and split by the bypass capacitor disposed in the vicinity of the IC or LSI.
The effect of suppressing diffusion of the high frequency power source current in a printed board to which the prior art shown in FIG. 5 to FIG. 7 is applied is shown in FIG. 8. In FIG. 8, the magnetic field distribution in the vicinity of a substrate is shown, with a stronger magnetic field being a darker color.
With the conventional example shown in FIG. 8(a), since the power source layer is a substrate comprising a flat board over the whole face, the high frequency power source current diffuses over the whole face of the substrate, and electronic equipment being a noise source, is shown in part with a particularly dark color. In the case of Japanese Patent No. 273447 shown in FIG. 8(b), since the power source layer is wired, diffusion of the high frequency power source current decreases, and it is shown that a common mode radiation from the electronic equipment is also suppressed.
This can be considered to be due to diffusion of the high frequency power source current from the IC/LSI being decreased by wiring the power source layer, and a strip line being formed by the power source wiring and the adjacent ground layer, thereby decreasing the electromagnetic coupling between the power source layer (line) and the signal line so that the common mode current decreases.
However, while the technique in the above described Japanese Patent No. 273447 is completely correct from a viewpoint of the conventional decoupling technique, in view of practical use, it has many problems.
A first problem is how to know the high frequency power source current generated with the high speed and high frequency operation. If this is not known, the decoupling inductor and the bypass capacitor cannot be designed. Basically, the circuit design is an operation for setting any two of the circuit voltage, current and impedance to an appropriate value. Particularly in the case of a digital circuit, only two states of xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d are used as the input/output signals. Therefore, circuit design has been performed by taking notice of the voltage alone, and the current and impedance have not been taken into consideration in the design. Hence, in fact, the characteristics of impedance and current have not been disclosed with respect to most semiconductor ICs and LSIs, ranging from those of worldwide standard to those specified by a customer, and it cannot be expected that these will be disclosed in the near future.
Therefore, methods for measuring the high frequency power source current are proposed not only for the semiconductor manufacturers but also for users, and movement towards global standardization is progressing. However, setting of the operating conditions and setting the measurement environment is relatively difficult. Hence, efficient measurement in the designing line is not easy. Therefore, at present it becomes necessary to estimate the power source current from available characteristic data, instead of the data for the high frequency power source current, allowing for some design error.
A second problem is that basically, parameters of the decoupling inductor and the bypass capacitor must be designed for each semiconductor IC or LSI. For high speed and high frequency operation of semiconductor ICs or LSIs, a circuit for efficiently splitting the high frequency power source current generated with the operation, to the bypass capacitor becomes necessary. It is a fundamental in the circuit design and a matter of course that this circuit is independently designed depending on the type of semiconductor IC or LSI and on changes in the working conditions. As described above however, such design has not heretofore been performed in the digital circuit. Therefore, in the short term the burden on the designer increases, and due to this increasing burden, the design period becomes longer, and design errors also increase. That is to say, in order to apply the decoupling design to a product design, sufficient preparation period becomes necessary for improving design tools, performing retraining of designers and so on.
FIG. 9 shows one example of characteristics of a high frequency power source current of the LSI. To solve the first and second problems described above, characteristics of the high frequency power source current as shown in FIG. 9 must be measured with respect to all ICs and LSIs mounted on a substrate, to determine an electric charge Q being an integral value of a waveform in one cycle, and a necessary capacity of the bypass capacitor must be determined, taking an allowable voltage fluctuation in each IC and LSI into consideration. Moreover, it becomes necessary to calculate a desired inductance value from an impedance ratio of the bypass capacitor to the decoupling inductor, and replace a wiring pattern length, to thereby design the wiring for the power source.
A third problem is that the material technology or manufacturing technology of the decoupling inductor and the bypass capacitor are lagging behind compared to the increase in speed and frequency of semiconductor ICs and LSIs. For example, the switching frequency of a CPU (Central Processing Unit) used for recent personal computers has increased up to about 500 MHz, and when high-speed switching is performed, higher harmonics of several GHz or higher are contained in the power source current of the semiconductor ICs or LSIs constituting the CPU.
However, with the present capacitor manufacturing technology, the resonance frequency of a capacitor having a capacitance of about 0.1 xcexcF required for the power source of the semiconductor IC or LSI remains within a limit of several tens of MHz. With frequencies higher than this, it will not act as a capacitor, but behaves as an inductor.
In order to operate in higher speed digital circuits in the future, improvement in the high frequency characteristics of the bypass capacitor will be essential. However, in the near future, there is little possibility of a large-capacity small-scale capacitor having a resonance frequency reaching an order of GHz, becoming available in the market. Also, for the decoupling inductor, unless there is progress with research and development on the structure and material, there is little possibility that an inductor having a resonance frequency reaching the GHz mark and an inductance of about several hundreds nH, as well as a current capacity reaching several A, will become available in the market in the near future.
As for the power supply circuit, it is necessary to pursue speed increases in the digital circuit in spite of the various problems as described above. Hence, a substitute measure which is considered relatively easy to put to practical use is required, at least for he time being.
In view of the above situation, it is a first object of the present invention to provide a multi-layer printed board having a direct current power source supply line structure which can be adopted even if a value of the high frequency power source current generated with the high speed and high frequency operation of a circuit is not disclosed, and even if the high frequency performance of a decoupling inductor and a bypass capacitor is not sufficient, and which enables the high speed and high frequency operation of a power source for semiconductor ICs and LSIs, without largely depending on the type and working conditions of semiconductor ICs and LSIs.
Moreover, it is a second object of the present invention to provide a multi-layer printed board having a direct current power source supply line structure that can suppress generation of a common mode noise due to high frequency power source current of semiconductor ICs or LSIs.
In order to solve the above problems, a multi-layer printed board according to the first aspect of the present invention comprises laminated ground layers through respective first insulating material layers on both upper and lower sides of a power source layer provided with power source wiring and a laminated signal layer provided with signal wiring through a second insulating material layer on one or both of the upper and lower sides.
According to the second aspect of the present invention, in a multi-layer printed board according to the first aspect, the first insulating material layer is comprised of a thin insulating material.
According to the third aspect of the present invention, in a multi-layer printed board according to the first aspect, the first insulating material layer is comprised of a film having a high dielectric constant.
According to the fourth aspect of the present invention, in a multi-layer printed board according to the first aspect, a ground layer is comprised of a conductive layer in a flat board shape over the whole face which does not include a cutout or independent wiring other than a through-hole and a wire hole.
According to the fifth aspect of the present invention, in a multi-layer printed board according to the first aspect, the power source wiring has either one of wider line width among a line width in which a voltage drop due to circuit current becomes less than a predetermined value and a line width in which the characteristic impedance of said power source wiring becomes less than a predetermined value.
According to the sixth aspect of the present invention, in a multi-layer printed board according to the fifth aspect, the power source wiring is comprised of an independent line structure provided between a direct current power source receiving terminal and each circuit element in the power source layer, and has a length longer than a length obtained by multiplying a wavelength of a high frequency component on the line, contained in the power source current for the circuit element by a value determined depending on termination conditions of the line.
According to the seventh aspect of the present invention, in a multi-layer printed board according to the fifth aspect, the power source wiring is comprised of a line pattern capable of accommodating the longest wiring having a constant width within a constant area.
According to the eighth aspect of the present invention, in a multi-layer printed board according to the seventh aspect, the line pattern is comprised of a meandering wiring.
According to the ninth aspect of the present invention, in a multi-layer printed board according to the fifth aspects, the power source wiring is connected with a capacitor between a connection point with the circuit element and the ground layer, and a capacitor between a direct current power source receiving terminal to which the power source wiring is connected and the ground layer.
According to a tenth aspect of the present invention, in a multi-layer printed board according to the ninth aspect, the power source wiring is terminated on the side of the circuit element, by a capacitor having a low characteristic impedance in the high frequency band of a high frequency component contained in the power source current, and is terminated on the side of the direct current power source receiving terminal, by a capacitor having a low characteristic impedance in the low frequency band of a high frequency component contained in the power source current.
According to the eleventh aspect of the present invention, in a multi-layer printed board according to the fifth aspect, a DC supply cable connecting the direct current power source receiving terminal and an external power source unit has a higher common mode impedance than that of the power source wiring.
According to the twelfth aspect of the present invention, in a multi-layer printed board according to the first aspect, the second insulating material layer is comprised of a glass epoxy resin board.
According to the thirteenth aspect of the present invention, in a multi-layer printed board according to the first aspect, the second insulating material layer comprises a ceramic board.
In this invention, the reason for having a line structure of a low impedance with the power source layer inserted between ground layers is as follows. That is, an ideal form of the direct current power source for a high-speed and high-frequency circuit element such as an IC or LSI mounted on a printed board is that the inner impedance has a sufficiently small value over a wide frequency band and that such a power source is provided for each circuit element such as an IC or LSI. By having this, the high frequency power source current flows smoothly down to ground, with the high speed and high frequency operation of the circuit element such as the IC or LSI. As a result it becomes possible to suppress distortion in the signal waveform, and to eliminate mutual interference between circuit elements such as ICs and LSIs, due to deterioration of the voltage stability.
However, independent installation of a power source for each circuit element increases the number of circuit parts, resulting in an increase in equipment cost, as well as increase in equipment size. Moreover, there is also a problem in that the probability of failure of the equipment increases. Hence this is not always practical. Therefore, in the case of relatively small-scale electronic equipment, power sources having the same voltage are often combined into one, so far as there is no special need. That is to say, the direct current power source for the printed boards normally generates power in a unit independent of the printed board, and supplies power via an electric cable, with no consideration being given to the influence of high frequencies.
Therefore, it becomes necessary to arrange elements within the printed board, such that the direct current power source supplied in such a form to the printed board can be supplied to ICs and LSIs in a form close to the above described ideal form.
In order to distribute the direct current in a form close to the ideal form, two methods are considered One is a concept for decreasing the impedance of the power source as much as possible, while giving priority to independent installation of a direct current power source. The method for forming an impedance added circuit in the conventional power source layer (for example, see Japanese Patent Application, First Publication No. Hei 8-137904) is included in this. The other concept is for maintaining independence of the power source as much as possible, while giving priority to decreasing the impedance of the power source. The present invention is based on this latter concept.
According to the construction of the present invention, by constructing a multi- layer printed board in the above described manner, an ideal direct current power source can be supplied independently in appearance to circuit elements such as ICs and LSIs mounted on the printed board. Hence, constraint factors with respect to the high speed operation of the circuit elements such as ICs and LSIs due to the power source section can be eliminated, as well as enabling suppression of electromagnetic coupling between the power source supply line and the signal line on the printed board where high frequency current is flowing, and flow out of the high frequency current from the power source supply line of the printed board to the power source supply cable in the apparatus.
As a result, high speed and high frequency operation of the circuit element such as ICs and LSIs mounted on the printed board can be ensured, electromagnetic radiation from high-speed and high-frequency electronic equipment such as digital equipment and the like is suppressed, and resistant strength against electric or electromagnetic disturbance from the outside can be improved.